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His father, the then hereditary prince (who reigned from 1760 and died in 1785) had in 1747 left the family and soon converted Catholicism, and in 1755 formally ended his marriage. The young prince Frederick, together with his two elder brothers, were with their mother the Landgravine Mary and became fostered by Protestant relatives in 1747. Soon the family moved to Denmark to be guests of her sister Louise of Great Britain, who died in 1751. His two elder brothers married Danish princesses - their first cousins - in 1763 and 1766 respectively. They remained in Denmark, becoming important lords and royal functionaries. Only his eldest brother returned to Kassel, in 1785 when ascending the landgraviate.

He married Princess Caroline of Nassau-Usingen (4 April 1762 Biebrich - 17 August 1823 Offenbach), a remarkable heiress of a family which became extinct in male line. 1781 he bought the castle of Rumpenheim, Offenbach, from his brother Carl, and it became the family's seat. His descendants are known as the Hesse-Kassel-Rumpenheim branch of the House of Hesse, one of only two branches that survived to the present day.